In recent years, optical disc media such as a CD (Compact Disc) and a DVD (Digital Versatile Disc) have diversified into various types, and means for recording information in these media has also diversified. In particular, because the recording density of a DVD is higher than that of a CD, a waveform reproduced from the DVD disc is distorted under the influence of intersymbol interference.
For example, 8/16 modulation is used for the DVD so that a pit length is regulated to be any of a plurality of predetermined lengths. However, the length of a pit is not necessarily precise. When the waveform of a read signal is distorted by intersymbol interference, misreading of data is likely to occur so that the reliability of data is significantly degraded. Therefore, a wavelength equalization technology which improves the performance of such a high-density optical disc against intersymbol interference has become indispensable.
As an example of the waveform equalization technology, there is a partial response (PR) technique which performs waveform equalization after giving known intersymbol interference. In relation to a waveform equalization learning control technology for the DVD, there has been known a DVD reproduction method using a PRML (Partial Response Maximum Likelihood) method which uses a (2, 10) RLL (Run Length Limited) code in 8/16 modulation and has an asymmetric waveform equalizing function (see, e.g., Patent Document 1).
Patent Document 1: Japanese Laid-Open Patent Publication No. 2003-85764